In a commercial dishwasher there is a requirement to automatically control the addition of detergent into the dishwasher's wash tank. Starting with fresh water, the controller must control the detergent dispenser in such a way that it dispenses detergent to the-desired concentration (called the Set Point) for the dishwashing process. Further, the controller must cause the dispenser to add additional detergent as the dishwashing process runs due to dilution of the detergent concentration caused mainly by the flow of the fresh water used in the rinsing process into the wash tank. A conductivity cell located in the wash tank is commonly used to sense the detergent concentration and to send a feedback signal to the controller. The present invention pertains to controllers using such a feedback sensor.
Typically the entire control system derives its power from a transformer connected in parallel with the dishwasher's wash pump. The transformer converts the dishwasher's higher voltage (typically 115 v.a.c. to 240 v.a.c.) down to the controller's operating voltage of 24 v.a.c. In most commercial dishwashing systems, the chemical dispenser's controller only receives power when the dishwasher's wash pump is operating. This has several advantages and is the commonly accepted method of deriving power for the detergent dispenser and the dispenser's controller. The benefits are that it provides an interlock so that no detergent can be dispensed if the wash tank is not full of water (there is a wash tank sensor in most machines that prevents the wash pump from running on a empty tank) and being agitated. Using the same connection to perform the wash pump interlock function as well as to provide power to operate the controller keeps the installation as simple as possible, which is very important with this type of equipment.
The present invention addresses the problem of controlling detergent concentration closely about the desired set point with little overshoot or undershoot. It further addresses the problem associated with field set-up of the current controllers to avoid excessive overshoot, especially when dispensing a detergent that is either slow to mix with the wash tank water or exhibits wide variations in its feed rate over time.
Typical dispensers used for dosing detergent into commercial dishwashers do not allow true proportional control of the feed rate because they employ a simple ON/OFF type solenoid valve which controls the flow of water to a nozzle. The spray from the nozzle is directed at the detergent which is typically either in powder, tablet or "brick" form. The sprayed water then dissolves some of the detergent or simply carries the undissolved detergent along with the sprayed water as it falls, due the gravity, down the dispenser's feed tube and into the dishwasher's wash tank.
There are several problems associated with controlling such a dispenser. The controller receives its feedback signal from a conductivity cell located in the wash tank that can only measure detergent that has dissolved into the wash tank water. The dissolution time of detergents varies due to several factors: chemical composition, dispenser spray water temperature, water pressure, wash tank temperature and wash tank agitation. Further, the controller can not measure the amount of detergent that is in the feed line connecting the dispenser to the wash tank and somehow this must be taken into account to allow good control.
It should be noted that if a simple control function is used that allows the solenoid to remain ON until the set point conductivity is sensed in the tank, the end detergent concentration will typically overshoot the required amount by 50% or more. A 50% overshoot on the initial charging of the wash tank will result in approximately 25% more detergent being used over the course of the typical length dishwasher operation between wash tank refills than would be used if there were no overshoot.